There has been a long-felt need for rapidly and accurately sensing the magnitude and/or number of depression and/or protrusion defects, particularly for quality control, on surfaces which require utmost smoothness for precision operation, as, for example, in the case of high quality optical devices, e.g. mirrors and lenses, semi-conductor wafers, and the like. Hitherto such sensing has generally been done manually and visually by operator scanning with a microscope or electron microscope. Such procedures have been found to be both slow and insufficiently precise in providing the desired information.
Laser Doppler velocimeters (LDV) have recently been developed for determining the rate of fluid flow in wind and fluid tunnels by suspending small particles in the fluid and determining their velocity and size by means of the velocimeter. Such velocimeters generally comprise convergent laser beams of equal size, intensity, and frequency which produce a stationary interference fringe pattern within the zone of convergence, sometimes called the probe volume. The interference fringes are planes which are normal to the plane defined by the center lines of the two converging laser beams and parallel to the bisector of the converging beams. In operation, the apparatus is set up to determine visibility in terms of the AC/DC ratio of the scattered light. The fluid-borne particles move across the fringes in trajectories normal to the fringe planes and normal to the converging beam bisector, thus crossing the fringe zone from the peripheral region of least intensity through the center region of maximum intensity, and then through the region of decreasing intensity. For optimum signal and resolution, the scattered-light collecting optics must be focused at or near the geometric center of the probe volume and, because of the rapid movement of the particle across it, the scattered radiation due to the particle generally consists of only a short burst in the order of microseconds. The number density of the particles passing through the stationary fringe pattern vary with consequent variation in the visibility bursts. The number can not be determined except on the basis of a hypothetical visibility equation which assumes that all of the particles are of substantially the same predetermined size. Such laser Doppler velocimeters are described in detail in the article by W. M. Farmer, "Measurement of Particle Size, Number Density, and Velocity Using a Laser Interferometer," Applied Optics, Vol. 11, No. 11, Nov. 1972, pp. 2603-2612, and G. J. Rudd, U.S. Pat. No. 3,680,961.
In more recent development of the Laser Doppler Velocimeter, the art discloses the use of probe volumes in which the fringes are caused to move continuously in a direction normal to the fringe planes by employing converging laser beams of the same intensity but slightly different frequency, e.g., a frequency difference within the radio frequency band. Such shifting of the frequency of one of the beams can, for example, be produced by diffraction of an incident laser beam by means of an ultrasonic Bragg cell, which can be made to divide the incident beam into two diverging beam components of the same intensity, one nondiffracted component having the incident beam frequency and the other diffracted component having its frequency shifted by an amount equal to the Bragg cell frequency. The difference in frequency between the two beams (.DELTA.f) is within the radio frequency band. Since the two coherent light beams which leave the Bragg cell are diverging, it is required that the beams be converged by an appropriate optical system to form the desired interference fringe pattern. The moving fringe pattern moves at a rate equal to .DELTA.f, which in turn is equal to the Bragg cell frequency.
The moving fringe technique has been applied to the LDV primarily to provide a means for determining the direction of movement of the particles moving across the fringe planes. It provides no substantial improvement in determination of particle size. The application of single and two-dimensional Bragg cell systems to the LDV is disclosed in Chu et al, "Bragg Diffraction of Light by Two Orthogonal Ultrasonic Waves in Water", Appl. Phys. Lett., Vol. 22, No. 11, 1 June 1973, pp. 557-59; and W. M. Farmer et al, "Two-Component, Self-Aligning Laser Vector Velocimeter," Applied Optics, Vol. 12, No. 11, Nov. 1973, pp. 2636-2640.
W. M. Farmer, "Observation of Large Particles with a Laser Interferometer,"Applied Optics, Vol. 13, No. 3, March 1974, is primarily concerned with verifying experimentally certain of the theoretical predictions made by Farmer, Nov. 1972 supra. On page 616, col. 2, line 20, to page 618, col. 1, line 3, he concerns himself with experiments to test the validity of Eq. (28) in Farmer, Nov. 1972, as an assumed means for determining particle density from visibility. Eq. 28 (which is derived from Eq. (26), same reference) assumes that EQU V.sub.n = V/.sqroot.n
where V is the ensemble average one-particle visibility function for the size distribution of the n particles. Visibility V is defined in Equation 13 (same reference) as the AC/DC ratio. Farmer, March 1974, randomly mounts 10-15.mu. glass spheres onto a glass plate which he positions in the x-y plane of a moving fringe pattern adjusted to a .lambda..sub.s = 120.mu.. He checks the plates for regions where the scattered light indicates relatively high or low concentrations of particles positioned within the given cross-sectional fringe pattern area and then examines a stationary plate position with a spatially-stationary fringe spot. For an indication of concentrations which Farmer considers high or low, see FIG. 11, page 620 where the number of particles derived from 10 separate stationary glass plate visibility measurements vary from 43 .+-. 20 to 308 .+-. 148. Based on the Farmer approach, the number must be determined with prior knowledge of particle size and, according to Farmer, "(the visibility measurement is strictly valid only for monodisperse sizes)." It is clear, therefore, that Farmer is concerned only with attempting (within an exceedingly wide margin of error) to determine from a single integrated visibility signal the number of particles of essentially the same, known size within a given, spatially-stationary fringe pattern cross-sectional area. The difficulty of such an undertaking is obvious from such factors as the probabilty that the signals of at least a substantial number of the particles may be cancelled out by the random distribution which will place some in light and in dark areas of the fringe zone and signal attenuation produced by individual particle distance from the geometric center of the fringe pattern. It is clear, therefore, that Farmer, March 1974, does not and indeed cannot teach or suggest applicant's process or apparatus, and, in fact, given his requirement for preknown, monodisperse sizes, teaches away from it.
None of the available art recognizes or discloses the present invention, its principle of operation or its use in sensing the effective magnitude of unknown, differently-sized depression and/or protrusion defects, the number of such defects, or the geographical distribution (topography) of such defects on a high-precision, smooth surface, or the presence and size of scattered "rough" spots on the precision surface.